In capacitive distance or proximity sensors the distance between the sensor and an electrically conductive object is measured without contact by generating and measuring electric alternating fields. From the measuring signal functions can be derived, for example switching functions.
A disadvantage of the capacitive sensors known from prior art is that they can be influenced by interfering electric fields from the environment of the capacitive sensor or of the sensor electrodes of the capacitive sensor, which can have a negative impact on the result of the measurement. Another disadvantage consists in the fact that the measuring signal depends on the grounding conditions of the capacitive sensor on the one hand and on the object which approaches the capacitive sensor on the other hand. If the concrete grounding conditions of the capacitive sensor or of the object are not known, a correct measurement of the distance between the sensor electrodes and the object or of an approach of the object to the sensor electrodes of the capacitive sensor cannot be guaranteed.
To detect the distance or approximation of an object without contact various principles are known from prior art, which vary in the way of signal generation and measurement as well as the number of necessary sensor electrodes.
A first principle known from prior art for capacitive distance or approximation detection provides for the use of a capacitive distance sensor with only one electrode. In this measurement system the capacitance of the electrode in relation to the ground potential of the measuring electronics of the capacitive distance sensor is measured. If an object for example a user, approaches the sensor electrode, the capacitance at the sensor electrode increases, which can be measured and evaluated accordingly.
Another measurement system provides a capacitive distance or proximity sensor with two sensor electrodes. One sensor electrode is operated as transmitting electrode and the other sensor electrode as reception electrode. The electric alternating field emitted at the transmitting electrode is coupled into the reception electrode and measured by means of an electrical signal tapped at the reception electrode. In case of approach of a user to the sensor electrodes the alternating electric field formed between the transmitting electrode and the reception electrode changes, which can be measured and evaluated accordingly.
A still further measurement system known from prior art provides a capacitive sensor with three sensor electrodes. Here two electrodes are operated as transmitting electrode and the third electrode as reception electrode. The transmission electrodes are controlled in phase or anti-phase. At the reception electrode a sum of the alternating electric fields emitted at the transmission electrodes is measured. In case of approach of a user the alternating electric field coupled into the reception electrode, which results from the electric alternating fields emitted at the transmission electrodes, changes. The change of the alternating electric field coupled into the reception electrode can accordingly be evaluated.
The three measuring principles known from prior art have in common the two already mentioned problems, i. e. the measuring signal                can be disturbed by external interfering electric fields, and/or        depends on how the sensor electronics and/or the object approaching the sensor electrodes are grounded.        
In the state of the art one tries to reduce these problems by appropriate measures, for example by                analogue or digital filtration of the measuring signal, and/or        appropriate choice of the moment of measurement and/or by appropriate choice of the measuring cycle, and/or        processing additional information and/or evaluation of the working condition.        
These measures however require an increased and thus complex circuit expenditure of measuring electronics, which has a negative impact on the production of capacitive sensor elements and production costs. Moreover by these measures only the effect of external interfering fields or the grounding conditions of the sensor electronics or of an object approaching the sensor electrodes on the measuring signal is reduced, so that a correct evaluation of the measuring signal cannot be guaranteed.